In recent years, computers and networks have advanced remarkably, and many kinds of information such as text data, image data, audio data, and the like are handled on the computers and networks. Since these data are digitized, it is easy to form copies of data with equivalent quality. For this reason, in order to protect the copyrights of data, copyright information, user information, and the like are often embedded as digital watermark information (to be simply referred to as “digital watermark” hereinafter) in image data and audio data.
“Digital watermarking” is a technique for embedding another information, which is visually and audibly imperceptible to a human being, in secrecy in image and audio data by a predetermined process of these data. By extracting a digital watermark from image and audio data, the copyright information, user information, identification information, and the like of that data can be obtained. With such information, for example, persons who made illicit copies, and apparatuses used to form illicit copies can be traced from the illicitly copied digital data. In other words, digital watermarking can be applied to protection of the copyrights and the like of images, anti-counterfeit technology, various kinds of information recording, and the like.
Conditions required of such digital watermarking are as follows.
(1) Quality: Digital watermark information must be embedded to be imperceptible, i.e., with least quality deterioration of source digital information.
(2) Robustness: Information embedded in digital information must remain undisturbed, i.e., embedded digital watermark information must never be lost by editing or attacks such as data compression, a filter process, and the like.
(3) Information size: The information size of information that can be embedded must be able to be selected in accordance with different purposes of use.
These conditions required for digital watermarking normally have a trade-off relationship. For example, upon implementing robust digital watermarking, relatively large quality deterioration occurs, and the information size of information that can be embedded becomes small.
Taking a multi-valued still image as an example, digital watermarking can be roughly classified into two methods, e.g., a method of embedding in the spatial domain and a method of embedding in the frequency domain.
Examples of the method of embedding a digital watermark in the spatial domain include an IBM scheme (W. Bender, D. Gruhl, & N. Morimoto, “Techniques for Data Hiding”, Proceedings of the SPIE, San Jose Calif., USA, February 1995), G. B. Rhoads & W. Linn, “Steganography method employing embedded calibration data”, U.S. Pat. No. 5,636,292, and the like, which employ patchwork.
Examples of the method of embedding a digital watermark in the frequency domain include an NTT scheme (Nakamura, Ogawa, & Takashima, “A Method of Watermarking in Frequency Domain for Protecting Copyright of Digital Image”, SCIS' 97-26A, January 1997), which exploits discrete cosine transformation, a scheme of National Defense Academy of Japan (Onishi, Oka, & Matsui, “A Watermarking Scheme for Image Data by PN Sequence”, SCIS' 97-26B, January 1997) which exploits discrete Fourier transformation, and a scheme of Mitsubishi and Kyushu University (Ishizuka, Sakai, & Sakurai, “Experimental Evaluation of Steganography Using Wavelet Transform”, SCIS' 97-26D, January 1997) and a Matsushita scheme (Inoue, Miyazaki, Yamamoto, & Katsura, “A Digital Watermark Technique based on the Wavelet Transform and its Robustness against Image Compression and Transformation”, SCIS' 98-3.2.A, January 1998) last two of which exploit discrete wavelet transformation, and the like.
Also, as methods to be applied to a binary image such as a digital document formed by text, line figures, and the like, a method of manipulating spaces in a text part (Nikkei Electronics, Mar. 10, 1997 (no. 684, pp. 164-168), a method of forming a binary image using binary cells (density patterns) each consisting of 2×2 pixels (Bit September 1999/Vol. 31, No. 9), and the like are known.
These methods are designed as pairs of digital watermark embedding and extraction processes, and are basically incompatible to each other. In general, methods of embedding a digital watermark in the spatial domain suffer less quality deterioration, but have low robustness. On the other hand, methods that exploit the frequency transformation suffer relatively large quality deterioration but can assure high robustness. That is, these methods have different features, i.e., some methods can assure high robustness but have a small information size of information that can be embedded, others can assure high quality but have low robustness, and so forth. Also, the embedding methods used for multi-valued images cannot be applied to binary images in principle.
Color images, monochrome text images, line figures, and the like are often observed when they are displayed on a monitor display, and are often printed. Recently, prints with very high image quality can be created using not only a color copying machine but also an inexpensive printer such as an ink-jet printer or the like. In addition, since an expensive color image scanner, and convenient image processing software and image edit software which run on a personal computer have prevailed, an image can be scanned from a print with high image quality, and an image having equivalent quality in practical use can be reproduced. Furthermore, an image is scanned from a print with high image quality, and monochrome binary character images, line figures, and the like can be extracted and diverted.
Various digital watermarking methods are available in correspondence with features, especially, those of image data in which a digital watermark is to be embedded, and a suited embedding method differs depending on image data. In each of these plurality of methods, digital watermark embedding and extraction processes are paired, but the different methods are incompatible to each other. For this reason, an embedding method dedicated to a multi-valued image is used for multi-valued images, and that dedicated to a binary image is used for document images. Nowadays, however, digital images shown in FIGS. 5 and 6, on which a photo image taken by a digital camera, a document created using wordprocessing software, and the like are mixed, are often created and printed. A digital watermarking method which can effectively applied to such digital image on which a plurality of image regions with different features are mixed and its print has been demanded. In the following description, a plurality of images having different features will be referred to as “dissimilar images”, and an image on which dissimilar images are mixed will be referred to as a “mixed image”.
Also, it is demanded to protect the copyrights and the like of individual dissimilar images which form a digital image which is scanned from a print of a mixed image using an image input apparatus such as an image scanner or the like. In other words, it is demanded to protect the copyrights and the like of individual images (to be referred to as “partial images” hereinafter) which form respective parts of images obtained by scanning an image, which is exchanged using a print as a medium, by an image scanner or the like.
In recent years, as for security measures for office documents, the idea based on ISO 15408 has globally spread, and such technical field is becoming increasingly important in that respect. As one of security management methods of document information, various kinds of digital watermarking techniques mentioned above have been proposed and used.
Security management can be used for various purposes such as illicit copy prevention of data, prevention of leakage or tampering of important information, copyright protection of document information, billing for use of image data and the like, and so forth, and various digital watermarking techniques have been proposed in correspondence with those purposes. For example, as a technique for imperceptibly embedding watermark information in digital image data, a method of computing the wavelet transforms of image data and embedding watermark information by exploiting redundancy in the frequency domain (disclosed in Japanese Patent Application No. 10-278629), or the like is known.
On the other hand, a binary image such as a document image has less redundancy, and it is difficult to implement digital watermarking for such image. However, some digital watermarking methods (to be referred to as “document watermarking” hereinafter) that utilize unique features of document images are known. For example, a method of shifting the baseline of a line (Japanese Patent No. 3,136,061), a method of manipulating an inter-word space length (U.S. Pat. No. 6,086,706, Japanese Patent Laid-Open No. 9-186603), a method of manipulating an inter-character space length (King Mongkut University, “Electronic document data hiding technique using inter-character space”, The 1998 IEEE Asia-Pacific Conf. On Circuits and Systems, 1998, pp. 419-422, a method of handling a document image as a bitmap image expressed by two, black and white values (Japanese Patent Laid-Open No. 11-234502), and the like are known.
The above methods are characterized in that the user cannot perceive watermark information embedded in an image (to be referred to as “invisible watermarking” hereinafter). Conversely, a method of embedding watermark information which clearly indicating that the watermark information is embedded (to be referred to as “visible watermarking” hereinafter) is also proposed. For example, Japanese Patent Application No. 10-352619 discloses a method of embedding a reversible operation result of an original image and an embedding sequence by comparing the pixel position of the original image and the shape of a watermark image to be embedded so that the watermark information is visible to the user.
Digital watermarking basically aims at embedding of some additional information in image data itself, and protects an original image using the embedded additional information (e.g., prevention of unauthorized use, copyright protection, protection of tampering of data, and the like). In other words, digital watermarking does not assume any purposes for inhibiting the user from viewing an original image itself or for allowing only a user who has predetermined authority to copy data.
Protection of an original image is applied to the entire image. For this reason, the user cannot often view or copy even an image contained in a protected image, which need not be protected.